File:American malacological bulletin (1987) (17535908043).jpg

Title: American malacological bulletin
Identifier: americanmal4519861987amer (find matches)
Year: 1983 (1980s)
Authors: American Malacological Union
Subjects: Mollusks; Mollusks
Publisher: (Hattiesburg, Miss. ?) : (American Malacological Union)
Contributing Library: Smithsonian Libraries
Digitizing Sponsor: Biodiversity Heritage Library

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88 100 200 AMER. MALAC. BULL. 5(1) (1987) 300 ppm Ca~ 400 500 600
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15 10 £ 3 Bulinus truncatus Hydrobia aponensis Mercuria conf. & punica Calcium meq/l Fig. 4. Absolute calcium and magnesium concentrations and ratios in relation to gastropods collected at 53 sampling stations in southern Tunisia (1970 and 1971, both in spring). where only three or four eggs per mass is normal. Increased sulphate content more adversely affects egg laying than increased chloride content. Due to the lack of sulphate determinations in the field study, however, one can- not decide whether or not this anion limits distribution of Bulinus truncatus in Tunisia. Adjusting the Ca/Mg ratios by adding magnesium as a chloride, though evidently better tolerated, primarily does not permit a decision as to whether the decrease in egg lay- ing rates below the Ca/Mg ratio of 0.75/1 (Algeria) or 0.5/1 (Tunisia) was caused by an adverse Ca/Mg ratio or by the increased chloride concentration. In regard to the Ca/Mg ratios, field distributions (Fig. 4) clearly demonstrate that all Tunisian water samples lie in a range between 3.4 and 0.63. This does not reach the ex- perimentally determined value found to form the threshold for "normal" Bulinus reproduction (Fig. 2). An effect of increased chloride concentration, using the same chloride amounts as in the Ca/Mg ratio variation, but a constant Ca/Mg ratio of 3.65/1, on the other hand, clear- ly shows that the significant drop of egg laying as well as eggs/mass lies between 334 and 866 ppm chloride. The highest field value in Bulinus habitats, 602 ppm, is in the same range. The upper limit in West Lybia, as found by Vermeil etal. (1952) (quoted by Deschiens, 1954), is in the same order 10 < tSi o LU 5 10 20 30 40 50 75 100 PER CENT WATER FROM GABES IN MIXTURE WITH DEIONIZED WATER Fig. 5. Egg laying rates of Bulinus truncatus in original water from a Bulinus free irrigation canal north of the oasis of Gabes and in a series of dilutions. Mean values of counts over three weeks. Other conditions as in figure 2. Differences are significant (t-test: p< 0.05) between 75 and 50% and between 50 and 10%.

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